Absorbent glass mat

A valve-regulated lead-acid battery (VRLA battery) sometimes called sealed lead-acid (SLA), gel cell, or maintenance free battery. Due to their construction, the Gel and Absorbent Glass Mat (AGM) types of VRLA can be mounted in any orientation, and do not require constant maintenance. The term "maintenance free" is a misnomer as VRLA batteries still require cleaning and regular functional testing. They are widely used in large portable electrical devices, off-grid power systems and similar roles, where large amounts of storage are needed at a lower cost than other low-maintenance technologies like lithium-ion.

There are three primary types of VRLA batteries, Sealed VR wet cell, AGM and Gel. Gel cells add silica dust to the electrolyte, forming a thick putty-like gel. These are sometimes referred to as "silicone batteries". AGM (Absorbent Glass Mat) batteries feature fiberglass mesh between the battery plates which serves to contain the electrolyte. Both designs offer advantages and disadvantages compared to conventional batteries and sealed VR wet cells, as well as each other.

Lead-acid cells consist of two plates of lead, which serve as electrodes, suspended in an electrolyte consisting of diluted sulfuric acid. VRLA cells have the same chemistry. "Wet cell" type VRLA's contain acid in liquid form. In AGM and Gel type VRLA's, the electrolyte is immobilized. In AGM this is accomplished with a fiberglass mat; in Gel batteries or "Gel cells", the electrolyte is in the form of a paste-like gel created by adding silica and other gelling agents to the electrolyte.

When a cell discharges, the lead and diluted acid undergo a chemical reaction that produces lead sulfate and water (see lead–acid battery for details of the chemical reaction). When a cell is subsequently charged, the lead sulfate and water are turned back into lead and acid. In all lead-acid battery designs, charging current must be adjusted to match the ability of the battery to absorb the energy. If the charging current is too great, electrolysis will occur, decomposing water into hydrogen and oxygen, in addition to the intended conversion of lead sulfate and water into lead dioxide, lead, and sulfuric acid (the reverse of the discharge process.) If these gases are allowed to escape, as in a conventional flooded cell, the battery will need to have water (or electrolyte) added from time to time. In contrast, VRLA batteries retain generated gases within the battery as long as the pressure remains within safe levels. Under normal operating conditions the gases can then recombine within the battery itself, sometimes with the help of a catalyst, and no additional electrolyte is needed [1]. However, if the pressure exceeds safety limits, safety valves open to allow the excess gases to escape, and in doing so regulate the pressure back to safe levels (hence "Valve-Regulated" in "VRLA").

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Wikipedia